Method for controlling residual stresses in metal



April 28, 1964 s. NACHTMAN ETAL 3,130,614

METHOD FOR CONTROLLING RESIDUAL STRESSES IN METAL Filed Oct. 5, 1957 3Sheets-Sheet l FIG-Z fggaz Roll 1 W F Roll 5 E i 3: LE J 5 INVENTORS511702 5 7hc/ziman BY fiaialislifilnkowskz dziorneys United StatesPatent 3,130,614 METHUD FUR CGNTROLLDIG RESIDUAL STRESSES IN IVETALEliiut S. Nachtman, Park Forest, and Natalis H. Polaltowski, Wilmette,IlL, assignors to La Salle Steel Co., Hammond, 11111., a corporation ofDelaware Filed Oct. 3, 1957, Ser. No. 688,047 4 Claims. (CI. 80-60) Thisinvention relates to the improvement of the residual stresscharacteristics of metals during processing in a cold finishingoperation and it relates more particularly to the control of residualstresses in steel by a rolling operation which can be employed in thecold finishing of such steels.

This invention has application to steels and other wrought metals ofvarious structural forms, such as bars, rods, wires, tubes and the like,and of various structural shapes, such as rounds, squares, rectangles,triangles, hexagons, and the like. While the concepts of this inventioninclude metals other than steel, the most important industrially is theapplication of the inventive concept to steels and this invention willhereinafter be described with reference thereto.

Before entering into a discussion of the invention, it should be pointedout that in the usual practice for the cold finishing of steels, as in adrawing, rolling or an extrusion operation, high residual stresses areleft in the steels. These high residual stresses are detrimental in manyof the applications and uses that are to be made of the steel. Theymanifest themselves in the warpage of parts that are formed thereof tomake the part unfit for its intended use. These residual stresses aregenerally tensile at the surfaces of the steel and they may cause cracksto develop in the metal during subsequent machining or headingoperations. When selected portions are removed from the steel, thestress balance in the steel is disturbed to the extent that the finishedproduct may become seriously distorted.

Various attempts have been made in the industry either to reduce theintensity of the stresses existing in the steel or to control theintensity and character of he stresses developed in the steel during thecold reduction or other operational steps. The former is represented bythe practice of heat treating or annealing the steel subsequent to coldfinishing to reduce the stresses that have been developed in the steel.Such heat treating or annealing operations are costly especially fromthe standpoint of the time and labor consumed, the equipment and spacerequired, and the large inventory required because of the tie-up ofmaterials in process. The latter is represented by the very recentlydeveloped process of elevated temperature reduction as described andclaimed in one of the applicants recently issued US. Patents 2,767,835,2,767,836, 2,767,837, and 2,767,838.

The latter is also represented by the invention recently made by Kyle etal. as described in the copending appiication Serial No. 484,726, filedJanuary 28, 1955, and now abandoned, wherein die shape and particularlya slight taper in the bearing portion of the die was found beneficiallyto affect the residual stress characteristics of bars or steel barswhich are drawn or extruded therethrough.

lt is an object of this invention to provide a still different methodfor the reduction or control of residual stresses existing in metals andit is a related object to pro duce a new and improved steel producthaving new and difierent residual stress characteristics. Morespecifically, it is an object of this invention to provide a methodwhich may be employed in the cold finishing of steel to minimize theresidual stresses present in the steel and to conice trol the intensityand direction thereof thereby to produce a steel having new and improvedphysical and mechanical properties.

These and other objects and advantages of this invention willhereinafter appear and for purposes of illustration, but not oflimitation, an embodiment of the invention is shown in the accompanyingdrawings in which- FIG. 1 is a schematic diagram of the arrangement ofparts for carrying out the invention;

FIG. 2 is a pictorial representation of a series of steel test barsillustrating the results secured in the practice of this invention;

FIG. 3 is a pictorial representation of a series of bars of the typeillustrated in FIG. 2 which further illustrates the results secured bythe practice of this invention;

FIG. 4 is a pictorial representation of another series of barsillustrating the results capable of being achieved by this invention;

FIG. 5 is a curve relating flare to percent reduction in edge-rolledbars;

FIG. 6 is a curve which relates flare to percent reduction infiat-rolled steel bars;

FIG. 7 is a curve similar to that of FIG. 5 relating warpage factor topercent reduction, and

FIG. 8 is a curve similar to that of FIG. 6 relating warpage factor topercent reduction.

It has been found that the high residual stresses developed in steelbars and rods during cold reduction, as in a drawing, extrusion orrolling operation, and which are generally tensile in character in thesurface or peripheral portions of the bar, can be markedly reduced oreliminated or directionally modified by subsequently processing thesteel in a rolling operation to take a final reduction in an amountgreater than 0.01 percent up to about 0.20 percent and preferably Withinthe range of 0.02 to 0.14 percent.

It has been found further that reduction or elimination of residualstresses by the described rolling operation is directional in that therelief of stresses Will be in a plane perpendicular to the surfacerolled with little, if any, effect on the residual stresses in planesparallel thereto. As a result, it becomes possible to achievedirectional control of stresses to produce steels having completely newand improved characteristics.

It was found further that subsequent treatment of the steel in otherplanes can be employed to provide the same desirable modifications ofresidual stresses in the other planes without disturbing the stressrelationships previously developed by the first pass or passes therebyto enable the principles of superposition to be employed for effectingcontrol of stresses in a steel.

The concepts of this invention can best be described by reference to theresults secured in stress tests which have been applied to steel squaresand flats processed in accordance with the process of this invention.

As used herein, the term warping value is an indication of theconcentration and character of the longitudinal stresses present insteel. In the test for warping, test pieces were slotted through theircenters for a distance of about 4 inches. The length of the slot Wasrecorded and the dimension perpendicular to the slot was also measured.The difference between the dimension of the piece before slotting andafter slotting represents the flare that is caused by the presence ofresidual stresses. The flare is considered positive, indicating tensilestresses predominating in the material, if the bar expands on slotting.The flare is considered negative, indicating the presence ofpredominating compressive stresses in the material, if the ends movetowards the cut through the center. The warping values determined War in100 originalthieknessXflare g I (slot length) 7 i The steel test barshaving a crosswise dimension of 1 inch and a thickness of /2 inch wereadvanced linearly between a pair of rollers 10 with the fiat faces (x-xin FIG. 2) in engagement with the periphery of the rollers. The rollerswere adjusted to different settings for applying different pressures tovary the amount of reduction taken during advancement of the steelthrough the rollers. The following is a tabulation of the percentreduction for each of the test bars as illustrated in FIG. 2 of thedrawings:

Percent reduction Test bar: (in inches) A (As drawn) B 0.04

The test bars were slotted lengthwise through the center between thefaces that were engaged by the rollers for a distance of 4 inches fromone end. The deflection of the arms may be taken as illustrative of theresidual stresses existing in the bar. A spread or increase in thespaced relationship between the ends is indicative of a predominance oftensile stresses and convergence of the ends may be taken as indicativeof the predominance of compressive stresses in the peripheral layers ofthe steel bars.

It may be seen from FIG. 2 that the magnitude of the residual stressesgauged by the warpage or flare of the bars slit along their centers inthe horizontal plane of symmetry of the roll pass, alters after a singlepass between the rotating cylindrical rolls and that the change ischaracteristically related to the pressure to which the bars have beensubjected in the roll pass or the percent reduction resulting therefrom.

From the degree of flare in the as drawn bar A, it will be apparent thatall of the bars prior to the roll pass were characterized by highresidual stresses at the surface which were predominantly tensile incharacter. Upon rolling with increased pressure to take correspondinglyincreased reductions in cross-section, the positive tensile stressesdecreased, as evidenced by specimens B and C, until a nearly zero stresslevel was reached, as evidenced by the specimen D. When the pressuresacting on the bar are further increased to efiFect a correspondinggreater reduction, the (positive) longitudinal tensile stresses whichare present at and below the bar surface become changed to a slightlycompressive or negative stress, as evidenced by the convergence whichtakes place in specimen E. Thereafter, the stress characteristics againrevert to positive tensile stresses, as evidenced by specimen G, afterfirst passing back through a neutral zone, as evidenced by specimen F.

From the foregoing, it will be apparent that the stress levels and thatthe type of stresses in a bar can be changed by a superficial rollingoperation materially to reduce the magnitude and distribution ofresidual stresses origi nally existing in the bar or substantially toelminate the predominance of the tensile stresses in certain portionsthereof or change the tensile stress characteristics to compressivestresses for negative warpage value simply by the process of compressingthe bar in a rolling operation to take a reduction within apredetermined narrow range in an amount from 0.01 to 0.2 percent,depending upon the residual stress levels and stress-straincharacteristics desired.

By way of further illustration of the concepts of this invention, barshaving their stress levels reduced to zero warpage value in onedirection, as by taking a pass be tween the rolls with a setting toproduce the conditions represented by bar D of FIG. 2, are passededgewise between the cylindrical rolls at various settings to processthe bars at various pressures and take corresponding reductions. Thebars illustrated in FIG. 3 have been edge-rolled to take the followingreductions:

A No reduction From the results illustrated in FIG. 3, it will beapparent that while the stresses have been neutralized in the xxdirection, as represented by bar D in FIG. 2, tensile stresses stillexist in the crosswise direction, as indicated by the spread that isillustrated in specimen A of FIG. 3. Application of pressure to takecorrespondreductions at first causes elimination of some of the tensilestresses, as illustrated by specimen B in FIG. 3, until a bar having thestresses neutralized is secured with a 0.04 percent reduction, asrepresented by specimen C in FIG. 3. Thereafter, as in the flat rolledbars, the stress levels again rise from neutral upon further increase inpressure and in the amount of reduction taken during the rollingoperation until tensile stresses exceeding those originally present inthe bar are developed, as illustrated by specimen E of FIG. 3.

The directional relationship between the stress modification and thedirection of applied pressure for reduction is illustrated by takingbars corresponding to bar D of FIG. 2 by slitting the bar centrallybetween the side edges in a direction crosswise to the direction ofroll. It will be seen from specimen A of FIG. 3 that a high stressrelationship dominated by tensile stresses still exists in the crosswisedirection while being reduced to about zero in the direction of the rollpass.

From these results it will be apparent that each rolling pass willaffect essentially only the residual stresses in a plane perpendiculartherewith to decrease therewith to decrease the stress levels to aboutzero followed by increase in the tensile stresses as the pressure orreduction exceeds the optimum for obtaining the zero stress levels. Theresults secured are similar to those secured with the specimens of FIG.2 with the exception that the stresses affected are perpendicularthereto without corresponding effect upon the stress conditions existingin the longitudinal direction but acting on planes normal to the former.

Thus to reduce the stress levels in structural shape, such as flats orsquares, it is essential to both edge-roll and flat-roll the steel. Thisis further illustrated by the square bars of FIG. 4 wherein the steelbars were passed between the rolls both fiatwise and edgewise. bar B,slotted diagonally, showed no flare as compared to the large flare inthe as drawn bar A. Thus the initially large stresses in the steel barhave been reduced by the two passes at right angles to each other toproduce a practically stress-free or dead bar.

A similar procedure has been applied to inch bar stock of hexagonalshape. To process the bar for production of stress levels in alldirections, it was found necessary to advance the bar for roll passes onall three parallel faces by turning the bar through an angle of degreesbetween each pass. It was found that the magnitude and the size of thethree residual stress systerns acting upon the planes parallel to thethree faces will be interdependent. Rolling the bar on one pair ofplanes operates to modify the residual stress distribution in the othertwo sets of planes under 60 and 120 degrees to the plane being worked.By proper selection of the rolling pressure in the three successivepasses, it is possible to reduce the initial residual stresses to afraction of their original value thereby materially to improve thedimensional stability of the bars. By proper control,

The rolled it is possible to also end up with a bar having slightcompressive stresses or negative warpage value.

The amount of pressure and proportional reduction in cross-section inthe rolled direction for reduction of stresses to the point wheretensile stress levels begin to rise again to undesirable levels willvary slightly from steels of one composition to another but, in anyevent, the pressure and the amount of reduction for efiecting thedesired results will usually reside within the range for effecting areduction of from 0.01 to 0.2 percent, and preferably from 0.02 to 0.14percent.

FIGS. and 6 relate the amount of flare to percent reduction in 1018steel bars of 1 x A2 inch which have previously been subjected to a coldreduction in a drawing operation. FIG. 5 charts the flare secured whenthe bars are rolled flatwise, as indicated by the included sketch, andFIG. 6 charts the flare as against percent reduction when the bars arerolled edgewise.

The curves of FIGS. 7 and 8 graphically support the visual presentationmade in FIGS. 2-4 and the flare values given in FIGS. 5 and 6.Commencing at about 0.02 percent reduction in the flat rolled and in theedge rolled bars, the warpage factor begins to fall from a relativelyhigh positive value, indicative of high tensile stresses, to lowervalues as the pressure is increased on the metal to increase the percentreduction. The warpage values charted against percent reduction in FIGS.7 and 8 fall off rapidly to a minimum at about 0.045 per cent reductionin the edge rolled bars and at about 0.1 percent reduction in the flatrolled bars. Beyond this minimum, further increases in pressure withcorresponding further increases in percent reduction causes the trend ofthe warpage values to reverse and rise again until the original warpageis exceeded at about 0.1 percent reduction in the edge rolled bars andat about 0.14 percent in the flat rolled bars.

It will be apparent that the warpage values can be made to approach thezero levels in the edge rolled bars and to drop below the zero level inthe fiat rolled bars to produce bars having a predominance ofcompressive stresses. The ability to enter into the range of compressivestresses of negative warpage values is not dependent upon thedirectional sequence of the rolling operations but is dependent moreupon the composition of the steel, the dimensional characteristics ofthe metal and the stresses originally present in the metal beforerolling. What is intended to be established is the remarkable effectthat a smfll reduction has, when achieved by a rolling operation, on theresidual stress characteristics of the steel and the ability to make useof such small reductions in a rolling step to reduce or to control theresidual stresses in the steel and the directional characteristicsthereof.

As illustrated by the curves, the shape of the curve or the position ofthe curve can be shifted slightly to the left or to the right or varieddepending upon the chemical composition of the metal, the dimensionalcharacteristics of the metal and its previous heat treating and forminghistory. In general, however, the desired results will be secured byrolling to secure a reduction within the area of 0.01 to 0.2 percentreduction, as previously pointed out.

While the inventive concepts can be employed with metals other thansteel, as represented by copper, brass, aluminum and alloys thereof, itwill be understood that fullest response will be experienced with steelsand especially non-austenitic steels characterized by having a pearliticstructure in a matrix of free ferrite.

From the foregoing, it will be apparent that a new and simple means hasbeen provided for the control and reduction of warpage values in steelthereby to make steels available having improved stress and warpagecharacteristics. It will be evident further that, in addition to stressreduction, the concepts of this invention can be used to control thedirectional stresses by taking roll passes in one direction as comparedto others or by taking an amount of reduction which will convert tensilestresses into a predominance of compressive stresses, thereby to producesteels having new and improved characteristics.

It will be understood that changes may be made in the details of themeans and equipment for effecting the desired reduction by rollingwithout departing from the spirit of the invention, especially asdefined in the following claims.

We claim:

1. In the processing of steel having a shape other than round and havingat least one pair of parallel faces, said steel originally containingresidual stresses, the modification of the intensity and direction ofthe residual stresses in said steel comprising the steps of advancingthe steel with a pair of parallel faces in a cold finishing operationbetween parallel faces of a pair of rolls in a rolling operation, andcontinuously compressing the steel between the parallel faces of saidroll dies during advancement of the steel therethrough to take a 0.02 to0.14 percent reduction in cross-sectional area whereby the stressesnormal to the roll passes are reduced, and advancing the steel betweenthe parallel faces of the rolls in a rolling operation atcircumferential angles with respect to the previous roll passescomprising a multiple of the number of faces divided into 360 degrees totake a 0.02 to 0.14 percent reduction in cross-sectional area to reducethe intensity and direction of residual stresses interdependently in thesteel.

2. In the processing of an elongated steel of polygonal shape having atleast one pair of parallel faces, said steel originally containingresidual stresses, the modification of the intensity and the characterof the residual stresses in the steel comprising the steps of advancingthe steel with a pair of parallel faces between the parallel faces of apair of rolls in a cold finishing operation, and continuouslycompressing the steel between the parallel faces of said rolls duringadvancement of the steel therethrough to take a 0.02 to 0.14 percentreduction, rotating the steel through an angle corresponding to 360degrees divided by the number of faces, advancing the steel again withanother pair of parallel faces between the parallel faces of the rollsto take a similar reduction, and continuing the rolling operationbetween the parallel faces of the rolls until the steel has been turnedthrough an angle not greater than 360 degrees.

3. In the processing of an elongated steel of polygonal shape having atleast one pair of parallel faces, said steel originally containingresidual stresses, the modification of the intensity and character ofthe residual stresses in the steel comprising the steps of advancing thesteel with a pair of parallel faces between the parallel faces of a pairof rolls in a cold finishing operation and continuously compressing thesteel between the parallel faces of said rails during advancement of thesteel therethrough to take a 0.02-0.14 percent reduction, rotating thesteel through an angle of at least degrees but less than degrees,advancing the steel again with another pair of parallel faces betweenthe parallel faces of the rolls to take a similar reduction, andcontinuing the rotation of the steel followed by the rolling operationbetween the parallel faces of the rolls until the steel has been turnedthrough an angle not greater than 360 degrees.

4. In the cold finishing of steel having a shape other than round andhaving at least one pair of parallel faces, said steel originallycontaining residual stresses, the modification of the intensity andcharacter of the residual stresses in the steel comprising the steps ofadvanc ing the steel in a final finishing operation with a pair ofparallel faces between parallel faces of a pair of rolls in a rollingoperation, and continuously compressing the steel between the parallelfaces of said rolls during advancement of the steel therethrough to takea 0.02-0.14 percent reduction in cross-sectional area.

(References on following page) References Cited in the file of thispatent UNITED STATES PATENTS Williams, Feb. 17, 1891 Fisk June 20, 1933Morris Dec. 19, 1944 Wise Feb. 20, 1945 Heller Feb. 22, 1949 Heller Aug.1, 1950 Sims Mar. 18, 1952 Appel July 14, 1959 FOREIGN PATENTS GreatBritain Feb. 11, 1935 8 OTHER REFERENCES Metals Handbook, 1948 edition,page 536. (Copy in Division 14.)

Making, Shaping, and Treating of Steel, by U.S. Steel, sixth edition,1951, pages 582, 1100-1107. (Copy in Div. 13.)

The Journal of the Institute of Metals, No. 2, 1914, vol. XII, page 27.(Copy in Scientific Library, tn-l-I59).

Surface Stressing of Metals, ASM, 1947, pages 85-142 (article by O. J.Horger). (Copy in Scientific Library, TA460-A44S.

Nachtman, E. 8.: Residual Stresses in Cold-Finished Steel Bars,Mechanical Engineering, volume 77, October 1955, pages 886889.

1. IN THE PROCESSING OF STEEL HAVING A SHAPE OTHER THAN ROUND AND HAVINGAT LEAST ONE PAIR OF PARALLEL FACES, SAID STEEL ORIGINALLY CONTAININGRESIDULA STRESSES, THE MODIFICATION OF THE INTENSITY AND DIRECTION OFTHE RESIDULA STRESSES IN SAID STEEL COMPRISING THE STEPS OF ADVANCINGTHE STEEL WITH A PAIR OF PARALLEL FACES IN A COLD FINISHING OPERATIONBETWEEN PARALLEL FACES OF A PAIR OF ROLLS IN A ROLLING OPERATION, ANDCONTINUOUSLY COMPRESSING THE STEEL BETWEEN THE PARALLEL FACES OF SAIDROLL DIES DURING ADVANCEMENT OF THE STEEL THERETHROUGH TO TAKE A 0.02 TO0.14 PERCENT REDUCTION IN CROSS-SECTIONAL AREA WHEREBY THE STRESS NORMALTO THE ROLL PASSES ARE REDUCED, AND ADVANCING THE STEEL BETWEEN THEPARALLEL FACES OF THE ROLLS IN A ROLLING OPERATION AT CIRCUMFERENTIALANGLES WITH RESPECT TO THE PREVIOUS ROLL PASSES COMRPISING A MULTIPLE OFTHE NUMBER OF FACES DIVIDED INTO 360 DEGREES TO TAKE A 0.02 TO 0.14PERCENT REDUCTION IN CROSS-SECTIONAL AREA TO REDUCE THE INTENSITY ANDDIRECTION OF RESIDULA STRESSES INTERDEPENDENTLY IN THE STEEL.